Transitions and loci of solubilization of nutraceuticals in U-type nonionic microemulsions studied by self-diffusion NMR

Abstract

U-type microemulsions based on five food-grade ingredients, water, R(+)-limonene, ethanol, propylene glycol, (or glycerol) and ethoxylated sorbitan esters (Tween 60 or Tween 80) were studied. The U-type phase diagram is characterized by a unique composition, fully dilutable with the aqueous phase that inverts progressively from an L₂ phase to an L₁ phase via a bicontinuous structure without phase separation. The ‘oil concentrate’ (surfactant, oil, alcohol) is loaded with water-insoluble solubilizates (phytosterols, lutein and lycopene) a few times more than the solubility capacity of the oil phase (without the surfactant). The maximum solubilization capacity (μ-value) was obtained for phytosterols and the minimum solubilization capacity was for lycopene. All solubilization values decrease with aqueous phase dilution. If solubilization is calculated on the basis of the oil content (α-value) or the oil + surfactant (γ-value) it is obvious that the interface plays a key role in the solubilization.The lipophilic solubilizates (by SD-NMR) are tightly packed and well accommodated at interfaces that convex (hydrophobic-in-nature) toward the water (water-in-oil microemulsions). Solubilization at the bicontinuous interface is lower and the solubilizates are loose packed. Once phase inversion occurs, and the interface becomes more hydrophilic and transforms into oil-in-water microemulsion, the solubilization becomes minimal, and most of the solubilizate desorbs. Phytosterols and lycopene induce the transition from W/O to bicontinuous and it occurs at lower water content (ca. 25 wt% aqueous phase in the presence of solubilizate vs. 35 wt% in its absence). The transition from bicontinuous to O/W is mostly unaffected (or undetected) since the interface flattens out and the solubilizate does not affect the curvature much. Lutein displays different behavior, the transition, from bicontinuous phase to O/W, occurs at higher water contents because its adsorption and packing are significantly tighter. Solubilization capacities of each of the nutraceuticals were determined for all dilution compositions.